Loss of lock (LOL) detection circuits are conventionally used in data communication transceivers to detect when a clock signal that is recovered from a received data stream deviates from a local reference clock frequency by more than a predetermined amount. The amount of the deviation is typically measured in parts per million (PPM). One known way to detect a loss of lock condition is to wait until the recovered clock and the local reference clock are phase aligned, and then measure how long it takes for a slip of one cycle to occur. This approach requires the following components: a circuit for detecting phase alignment of the clocks; two counters for keeping track of how many times each clock has cycled since the phase alignment; and a circuit for detecting when a slip of one cycle has occurred. Thus, if it is desired, for example, to implement a loss of lock detection circuit that can detect a 400 PPM deviation, the following components would be required: at least one phase detector; two counters that can count up to 2500 (because 400 PPM corresponds to a one cycle deviation in 2500 cycles); and control circuitry to indicate when phase alignment and cycle slip conditions occur. If it is desired to detect a 200 PPM deviation, this would require two counters that can count up to 5000 (a count of 5000 requires 13-bit counters), and the aforementioned phase detector and control circuitry.
It is therefore desirable to reduce the amount of circuitry required to implement a loss of lock detector, which would correspondingly reduce the space requirements and power consumption of the loss of lock detector.
The invention provides for detection of a loss of lock condition by examining conventionally available information indicative of a rate of change of an interpolation ratio according to which first and second phases of the local reference clock are combined to form the recovered clock. This use of already available information reduces the amount of circuitry required for loss of lock detection, and also provides for faster detection of a loss of lock condition.